does anyone know how much of a performance increase is gained by mirroring two hard drives using a RAID card? does XP support this?

It doesn't, if anything it slows it down. If u want performance u need disk striping.

Its not necessarily true that it slows down, it depends, it may speed up. Performance may slow down if your environment is write intensive. Since you are miroring, an image has to be written to both disks, however the controller may buffer the writes and notify the OS that the write has completed even if the data is still in the buffer, or it may notify that the write has completed when only one of the disks has been updated, or it may wait for both. In advanced RAID configurations the spindles of the disks may be synchrosised and the writes occurr in parallel. There may be a large buffer in the controller and I/Os may be pended and sorted meaning physical updates occur less frequently and in larger I/O sizes reducing the seek on the disk. Hardware mirroring in a controller is normally faster and more efficient than s/ware mirroring by the operating system. Also dont forget that some controllers in mirroring or RAID-1 to give it its other name, do read balancing and alternate reads from both halves of the mirror, which increases read performance ABOVE that of RAID-0 (striping with no redundncy) which contains only 1 image of the data. RAID-0 can speed up I/O by parallel reads, however this only generally applies in a large sequential read environment, such as database sequential processing or perhaps reading large sound or video files. Small random reads are better from a mirrored pair since when reading from a stripe a read request normally generates an i/o of a whole stripe, which means transfering no of disks in stripe x stripe size bytes into the controller buffer irrespective of the size of the I/O request from the operating system, hence if you're going to do this large I/O across the stripe and only use a fraction of the data as in a random read, you're introducing a large perfromance hit. In large heavy use random I/O systems software mirroring of RAID/0 stripes can aid performance by balancing I/O across multiple spindles hence reducing hotspots, the large no of random I/Os are sorted by the raid controller and grouped together into larger contiguous(next to each other on the disk) I/Os again reducing seek. You wouldnt run RAID-0 by itself in a commercial environment due to single point of failure, unless you can live with the downtime to replace and restore. Another option RAID-5 gives you a cheap option for redundancy by striping parity across all disks in a stripe however suffers badly in write intensive environments unless there is a large I/O cache buffer. I used to make a living out of tuning systems. Paul